This invention relates to micromachined silicon diaphragms and specifically to methods of manufacturing such diaphragms.
The principle of working of many micromechanical devices employs the use of flexible diaphragms as a flexural part, usually acting as a passive transducing element. The wide range of devices incorporating flexible diaphragms includes micromachined pressure sensors, microphones and a variety of microfluidic devices such as micropumps and ink-jet print-heads.
The geometry tolerance of the diaphragm during the fabrication process, as well as its thermal compatibility with the rest of the device can have a great impact on the overall device performance, especially in applications such as low pressure sensing or precise pico-litre volume liquid handling.
Since the beginning of the micromachining era, different solutions have been employed in terms of material and geometry control of the diaphragms. Pressure sensors from their early stage employed thin silicon diaphragms as the sensing element. Diaphragms were formed by simply anisotropically etching exposed silicon areas with the thickness of the diaphragms being controlled either by timed etching or by etch-stop techniques such as heavy boron doping or reverse p-n junction formation.
The flexural element material in ink-jet print-heads and micropumps was usually made of stainless steel, glass or silicon.
An example of a stainless steel diaphragm is shown in xe2x80x9cThe piezoelectric capillary injectorxe2x80x94A new hydrodynamic method for dot pattern generationxe2x80x9d, E Stemme and S. Larsson, IEEE Transactions on Electron Devices, Vol. ED-20, No. 1, January 1973, pages 14-19.
Examples of glass diaphragms are shown in xe2x80x9cFabrication of an integrated planar silicon ink-jet structurexe2x80x9d, K. Petersen, IEEE Transactions on Electron Devices, Vol. ED-26, No. 12, December 1979 and xe2x80x9cMicromachined flat-walled valveless diffuser pumpsxe2x80x9d, A. Olsson, P. Enoksson, G. Stemme and E. Stemme, Journal of Microelectromechanical Systems, Vol. 6, No. 2, June 1997.
Examples of silicon diaphragms are given in xe2x80x9cDesign and development of a silicon microfabricated flow-through dispenser for on-line picolitre sample handlingxe2x80x9d, T. Laurell, L. Wallman and J. Nilsson, Journal of Micromechanics and Microengineering 9 (1999), pages 369-376 and xe2x80x9cThe flow structure inside a microfabricated inkjet printheadxe2x80x9d, C. Meinhart and H. Zhang, Journal of Microelectromechanical Systems, Vol. 9, No. 1, March 2000, pages 67-75.
The choice of the diaphragm material is dependent on the compatibility with the overall fabrication process and in terms of standard micromachining technology based on batch fabrication two main materials are glass and silicon. The limitations of using glass consist firstly of the difficulty of its precise machining and secondly of its thermal mismatch with the silicon. These limitations disappear in the case of silicon.
In most cases silicon diaphragms are formed using silicon etching accompanied by etch-stop techniques. U.S. Pat. No. 4,872,945 describes a process for manufacturing such a silicon diaphragm where a cavity is selectively etched into one face of a silicon wafer prior to etching the other face in order to control the thickness of the wafer.
U.S. Pat. No. 5,915,168 describes a process for manufacturing a cover for an air bridge structure. However, the structure is formed in such a way that it is not flexible.
A significant disadvantage of using etching to reduce the thickness of a silicon wafer is the length of time taken. As an example, removal of 200 xcexcm of silicon by anisotropic etching using an aqueous solution of potassium hydroxide (KOH) in standard conditions takes three to four hours. Furthermore, the process is dependent on conditions such as temperature.
In accordance with one aspect of the present invention, there is provided a method of manufacturing a diaphragm over a cavity, the method comprising reducing the thickness of a wafer by grinding and providing a cavity under the diaphragm.
An advantage of grinding lies in its purely mechanical nature and time efficiency. Removal of 200 xcexcm of silicon using silicon grinding takes typically five minutes and is independent of temperature, and hence, it is possible to manufacture flexible silicon diaphragms with high precision and at high speed in this way.
The diaphragm may be manufactured over the cavity by reducing the thickness of the wafer by grinding one face and forming a cavity in the opposite face.
Alternatively, the cavity may be provided in a further wafer which is bonded to the one wafer prior to grinding the one wafer.
The cavity may be formed either prior to or subsequent to reducing the thickness of the wafer.
The cavity may be formed in a variety of ways including etching.
The diaphragm may be supported during grinding to prevent distortion of the diaphragm. Typically, this support is provided by a sacrificial layer which is removed after grinding.
In a preferred embodiment, the nascent cavity may be used as the sacrificial layer. In a silicon wafer, this may be achieved by forming porous silicon as the sacrificial layer.
The wafer used may carry only one diaphragm. However, for economic reasons, it is desirable for a plurality of diaphragms to be manufactured on the same wafer.
A drop-on-demand dispenser can be manufactured by manufacturing a diaphragm over a cavity according to the first aspect of the invention and bonding thereto a second wafer having a nozzle communicating with the cavity. A plurality of such devices may be fabricated on the two wafers.
The thickness reduction may be performed prior to the bonding step. However, in order to improve the ease of handling, the bonding step is typically performed prior to the thickness reduction step.
The wafers may be made of a variety of materials including glass but, in a preferred embodiment, they are made of a semiconductor such as silicon.
Depending on the application, various sensor or actuator structures could be formed on the diaphragm. These include, but are not restricted to, pneumatic, thermal, electrostatic, piezoresistive and piezoelectric devices. In the case of a piezoelectric device, this could be glued to the diaphragm. Preferably, however, top and bottom electrodes and a piezoelectric element are screen printed onto the diaphragm. This screen printing process is advantageously performed prior to removing the sacrificial layer.